A Physical Approach to Grain Refinement of Wrought Mg Alloys via Solidification Control

人类社会高度依赖地球气候，因为气候模式在很大程度上决定了我们是否会有足够的食物和淡水。全球气温的逐渐上升，主要是由于大气中有害气体二氧化碳的数量增加，可能会导致重大的气候变化。道路运输活动约占排放到大气中的人为二氧化碳总量的24%，乘用车约占这些排放的60%。因此，为了防止地球气候恶化，减少乘用车的二氧化碳排放是一个重要的问题。要做到这一点，最有效和最简单的方法之一是减轻汽车的重量，这样汽车就会消耗更少的汽油或柴油。镁是一种非常轻的金属，事实上，有些镁材料可以制作得比水还轻。另一方面，他们仍然足够强大，可以制造汽车中使用的大部分零部件。因此，它们对汽车制造商非常有吸引力。就像一座由许多沙子组成的沙堡一样，镁材料也是由许多小颗粒组成的。镁材料中的晶粒度在决定材料是否具有延性方面起着非常重要的作用。一般来说，晶粒度越小，镁材料的延展性越好。因此，镁材料具有非常小的晶粒度是非常有利的，这样我们就可以很容易地将它们加工成不同的形状，如板材、管、棒、棒等。这些形式的镁产品都非常有用，用于制造汽车零部件和玩具、自行车、计算机、手机、电视等零部件。不幸的是，大而厚的镁材料通常具有粗大的晶粒度，因此，它们的延展性不够。因此，人们不得不使用非常缓慢的制造工艺来制造镁板、管、棒、棒等。这使得它们非常昂贵，包括大多数汽车制造商在内的许多客户愿意大量使用它们。因此，减小它们的颗粒尺寸是至关重要的。类似于水变成冰的过程，固体镁材料开始时是液态镁。从液体到固体的变化称为凝固，它决定了镁材料的颗粒大小。通过有效地控制凝固过程，可以获得非常细小的晶粒度。在过去的65年里，人们为控制镁材料的凝固过程做出了许多努力。虽然已经有了一些积极的进展，但所产生的颗粒尺寸仍然不够小。在这个项目中，我们提出了一种独特的方法，称为双螺杆熔体剪切，它可以有效地控制镁材料的凝固。这种方法的关键是确保在凝固过程中尽可能多的细小颗粒在液体中存活。这是通过迅速将块状液体温度降低到临界值以下来实现的；在这样做的过程中，将获得非常细小和均匀的颗粒尺寸。初步实验给出了非常令人鼓舞和令人兴奋的结果，表明这一概念是可行的。因此，希望对这种新的凝固控制工艺的进一步研究将开发出一种非常有益的加工系统，该系统将能够在大而厚的镁产品中提供细小的晶粒度。预计这项新技术也将适用于其他材料的凝固，例如铝和钛。这项研究的预期结果将对全球社会产生环境和经济上的好处。

Human society is highly dependent on the Earth's climate, as climate patterns largely determine whether we will have enough food and fresh water. The gradual increase in the global temperature, due primarily to the increased amount of an undesirable gas, CO2, in the atmosphere, can cause significant climate changes. Road transport activities account for about 24% of the total man-made CO2 released to the atmosphere, and passenger cars are responsible for about 60% of these emissions. It is therefore an important issue to reduce the CO2 emissions from passenger cars in order to prevent the Earth's climate from deteriorating. One of the most efficient and easiest ways to do this, is to reduce the weight of a car so that the car will burn less petrol or diesel. Magnesium is a very light metal, in fact, some magnesium materials can be made lighter than water. On the other hand, they are still strong enough for making most of the parts used in a car. Therefore, they are very attractive to car manufacturers.Just like a sand castle that is made of many grains of sand, magnesium materials are composed of many small grains as well. The grain size in a magnesium material plays a very important role in determining whether the material is ductile or not. In general, the smaller the grain size, the more ductile the magnesium material will be. It is thus highly beneficial for magnesium materials to have a very small grain size, so that we can readily manufacture them into different shapes, such as sheet, tubes, bars, rods, etc. These forms of magnesium products are all very useful for making car parts and parts used in toys, bicycles, computers, mobile phones, televisions, etc. Unfortunately, large, thick magnesium materials normally have a coarse grain size, as a result, they are not ductile enough. Therefore, one has to use a very slow manufacturing process to make magnesium sheet, tubes, bars, rods, etc. This makes them very expensive and not many customers including most car manufacturers, are willing to use them in a large quantity. It is therefore crucial to reduce their grain size. Similar to the process of water becoming ice, solid magnesium materials start off as liquid magnesium. The change from liquid to solid is called solidification, which determines the grain size of a magnesium material. By effectively controlling the solidification process one can obtain a very fine grain size. In the past 65 years, there have been many efforts towards controlling the solidification process of magnesium materials. Although there have been some positive developments, the resultant grain size is still not small enough. In this programme, we propose a unique approach, designated 'twin-screw melt shearing'; it can effectively control the solidification of a magnesium material. The key point of this approach is to ensure that as many small grains as possible survive in the liquid during solidification. This is done by rapidly lowering the bulk liquid temperature to below a critical value; in doing so, the process will give a very fine and uniform grain size. Preliminary experiments have given very encouraging and exciting results, suggesting that the concept is feasible. Therefore it is hoped that further study into this new solidification control process will develop a hugely beneficial processing system, which will be able to deliver a fine grain size in large, thick magnesium products. It is further anticipated that the new technology will also be applicable to the solidification of other materials, such as aluminium and titanium.The anticipated results from this study will be both environmentally and economically beneficial to the global community.